The present invention relates to measuring signal degradation in communications systems. In particular, the invention relates to methods and apparatus for non-invasively measuring performance of a digital broadcast system.
Digital communication technologies offer numerous advantages over their analog predecessors. Nearly error-free transmission is guaranteed if a threshold signal-to-noise ratio is available. Channel distortions may be corrected using adaptive equalizers. Coding techniques may be advantageously employed both to overcome channel-related signal impairments and to minimize usage of bandwidth.
Nonetheless, digital communication system operation is not fault-free. The transmission of information over a physical medium ultimately requires analog components such as RF mixers, amplifiers, oscillators, etc. that are prone to misalignment, temperature caused drift, and various other modes of failure. The physical medium itself also introduces impairments in the form of added noise and reflections. Digital receivers can inherently correct for errors up to a certain threshold, but if that threshold is exceeded, communication is severely impaired.
Digital communication techniques found their earliest application in the context of point-to-point links as would be used by the military or commercial common carriers. In these applications, the link may be tested by interrupting normal service and transmitting test signals. For example, if it is desired to test the link margin of a link, a test signal could be transmitted at progressively reduced power levels until the signal can no longer be accurately received.
Increasingly, digital techniques are being applied to broadcasting. One important example is broadcasting of digital video signals to numerous subscribers over coaxial cable, or over the air. Commercial digital broadcast systems require high reliability yet these digital broadcast systems cannot be tested as easily as the point-to-point links. Interrupting normal service for transmission and measurement of test signals is not a commercially viable option.
The present invention provides method and apparatus for non-invasive testing of digital communications test systems. In accordance with a first aspect of the invention, amplitude measurements are made for multiple frequencies of a multi-frequency communication system, and converted to the time domain. An adaptive filter then seeks to match its output to the time domain representation, thereby characterizing the channel. Impedance mismatches may be precisely located using this technique.
In accordance with a second aspect of the invention, an error signal representing a difference between a signal transmitted through the channel and a received signal is estimated and analyzed. The error signal is separated into components corresponding to contributions by wide band noise, residual phase modulation, and residual amplitude modulation. Identification and removal of narrow-band interferers may occur prior to this separation. Bit error rate and system margin computations employ a Monte Carlo simulation that simulates the various error sources. This provides a well refined estimate of bit error rate and system margin.
In accordance with a third aspect of the invention, a cluster variance measurement based on an error signal is used to estimate bit error rate. In accordance with a fourth aspect of the invention, an error signal is displayed in the frequency domain relative to a measured power of a received signal.
The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings.